Level independent automatic cable equalizer

Abstract

A multiple section cable equalizer with a signal level detector produces a clipping control signal for amplifiers in each equalizer section, so that a variable level input signal is restored to an equalized output signal that is free of distortion and attendant bit errors. A fixed reference clipping stage after the last equalizing stage produces a signal clipped to a final output level. A feedback path restores DC and low frequencies lost due to AC coupling. A test output stage permits monitoring of the operation of the equalizer stages. The signal level detector contains (in series), a differential amplifier with low pass filtering, a balanced modulator detector, a voltage amplifier, a comparator and peak detector, a transconductance amplifier and a plurality of current mirrors operatively connected to determine a difference current from the output of the transconductance amplifier and convert this to a clipping control signal voltage level. A defeat comparator monitors the output of the comparator and peak detector and replaces the variable difference current with a fixed reference current when an external capacitor used by the peak detector appears to be shorted.

I claim: 
     
       1. An automatic cable equalizer for compensating for a variable level input signal with possibly degraded rise and fall times, the equalizer comprising: equalizer means, having an input and an output, with the input coupled to receive the variable level input signal, the equalizer means containing amplifying means, high frequency gain boosting means and clipping means, the clipping means having a clipping control input coupled to receive a clipping control signal;   signal level detecting means coupled to monitor the variable level input signal and for producing the clipping control signal, the clipping control signal being proportional to the peak level of the rectified low frequency content of the variable level input signal.   
     
     
       2. An automatic cable equalizer according to claim 1 further comprising: an AC coupling capacitor coupled to the equalizer input; and   feedback means having an input and an output, with the input coupled to receive the output of the equalizer section and with the output coupled to restore the DC lost by AC coupling.   
     
     
       3. An automatic cable equalizer according to claim 2, wherein an RC time associated with the AC coupling capacitor is approximately equal to an RC time associated with the feedback means. 
     
     
       4. An automatic cable equalizer according to claim 1 further comprising a limiting section having an input and an output, with the input coupled to receive the output of the equalizer section, the limiting section limiting to a fixed clipping level. 
     
     
       5. An automatic cable equalizer according to claim 1 further comprising a test output stage having approximately unity voltage gain, a high impedance input and a low impedance output, with the input coupled to the output of the equalizer. 
     
     
       6. An automatic cable equalizer according to claim 1, wherein the signal level detecting means comprises: low pass filtering means coupled to receive the variable level input signal and produce a low frequency content signal;   rectifying means coupled to receive the low frequency content signal and producing a rectified low frequency content signal;   peak holding means coupled to receive the rectified low frequency content signal and producing a maximum low frequency content signal; and   scaling means coupled to receive the maximum low frequency content signal and producing the clipping control signal proportionally to the maximum low frequency content signal.   
     
     
       7. An automatic cable equalizer according to claim 6, wherein the peak holding means comprises: charge storing means for holding a peak charge;   comparator means having a first input, a second input and an output, with the first input coupled to receive the rectified low frequency content signal and the second input coupled to receive an indication of the peak charge, the output indicating when the first input is larger than the second input; and   charge adding means responsive to the output of the comparator means for adding to the peak charge when the rectified low frequency content signal exceeds the present peak charge.   
     
     
       8. An automatic cable equalizer according to claim 6, wherein the scaling means comprises: transconductance amplifying means having an input and an output, the input being coupled to receive the maximum low frequency content signal and the output being a current proportional to the magnitude of the maximum low frequency content signal; and   signal producing means for receiving the output current from the transconductance amplifier and producing the clipping control signal proportionally to the received output current.   
     
     
       9. An automatic cable equalizer according to claim 8, wherein the transconductance amplifying means comprises: a transconductance amplifier having an input and two outputs, the input being coupled to receive the maximum low frequency content signal and the outputs being two currents whose difference is proportional to the magnitude of the maximum low frequency content signal; and   a plurality of current mirrors receiving the output currents from the transconductance amplifier operatively connected to establish the difference current.   
     
     
       10. An automatic cable equalizer according to claim 8, wherein the scaling means further comprises defeat comparator means having first and second inputs and an output, with the first input coupled to receive the maximum low frequency content signal and the second input coupled to receive a reference signal, and producing at the output a selection signal selecting the difference current or a fixed current as a basis for establishing the level of the clipping control signal.